1. Field of the Invention
The present invention relates to spark plugs for internal combustion engines and, more particularly, to a small-sized spark plug having a metal shell formed with a mounting thread of a value less than M10.
2. Description of the Related Art
In general, the related art spark plug is known to include a metal shell formed with a mounting thread to be mounted to an engine, a porcelain insulator fixedly secured to an inside of the metal shell such that one end of the porcelain insulator protrudes from one end of the metal shell, a center electrode fixedly secured to an axial bore of the porcelain insulator such that one end of the center electrode protrudes from the one end of the porcelain insulator, and a ground electrode fixedly secured to the metal shell and having one end placed in a face-to-face relationship with the one end of the center electrode by a spark discharge gap.
Recently, there is an increasing demand to provide an engine with a high power output in which a cylinder body is formed with an intake manifold and an exhaust manifold, associated with intake and exhaust valves formed in enlarged diameters, and a water jacket to provide improvement over delivery of coolant water.
For this reason, a need arises for decreasing an installation space occupied for a spark plug mounted in the engine and ensuring an increased space around a combustion chamber, and to this end, there is an increasing demand for the spark plug to be minimized (in a smaller diameter configuration).
In the related art, for instance, sizes of mounting threads of spark plugs have taken a standard metric thread of M14 on JIS (Japanese Industrial Standard). However, a need arises for providing miniaturized spark plugs each with a mounting thread formed in a smaller diameter less than M10 as disclosed in Japanese Utility Model No. 5-55490.
By the way, because of a demand for high power outputs of the engines described above, the engines have high compression ratios. To satisfy such a demand, the spark plugs are required to operate at increased discharge voltages (demanded voltages) and subjected to severe circumstances in order to ensure a withstand voltage.
Particularly, with an attempt to structure the spark plugs in a narrow diameter configuration as set forth above, the porcelain insulator, electrically insulating the center electrode and the metal shell from one another, results in a reduced wall thickness, causing important issues with an increased probability of decreasing the plug's ability to withstand voltage.
In the related art, dielectric breakdowns occur in the spark plugs in areas where the porcelain insulators and the metal shells are held in engagement and the spark plugs are sufficed to ensure the withstand voltage at those areas. However, due to severe operating circumstances of the engines recently in use, dielectric breakdowns also occur in the spark plugs even at other areas, raising a need to take new counter measure.